A great number of studies concerning on-line verification systems have been conducted by researchers in the last years. However, investigations on the influence of different signature sizes on the process of signature formation are little. In fact, this kind of analysis for on-line signature verification system is been made for the first time. In this paper, in order to investigate this influence, a database containing signatures of three different sizes was created. The experimental results show that signatures of different sizes are statistically different and they can influence the feature extraction techniques. Thus, a special attention has to be taken in the implementation of global applications that use databases with signatures of different sizes.